U.S. Pat. No. 5,439,420 to Ford Motor Company Limited discloses a belt tensioning device for automotive engines that has an arm pivotally mounted to the engine and pulley rotatably mounted to the arm for contacting the drive belt. The pulley is urged into contact with the drive belt by the arm. The tensioner includes a governor for controlling the rotational motion of the arm. The governor allows the arm to rotate freely in the direction in which the tension of the drive belt is increased and resists the rotation of the arm in the opposite direction where tension of the drive belt decreases. The problem with this device is that the governor is essentially a passive device and there is no assurance that the system will in fact reach a desired tension level.
U.S. Pat. No. 4,478,595 to Nippondenso Co., Ltd. discloses an idler pulley that is radially displaced under control of an actuator for applying a tensioning force on an endless belt of an accessory drive system. A microcomputer calculates drive power and torque necessary for driving the vehicle accessories and based on this calculates an optimum tensioning force for the belt which attempts to maximize the belt power transmission efficiency. The microcomputer calculates an actual tensioning force on the belt based on the compression of a spring and generates a control signal when the calculated actual tensioning force is different from the calculated optimum tensioning force. A drive circuit receives the control signal and operates the actuator in accordance with the difference between the calculated actual and optimum tensioning force. One of the shortcomings of this device is that the amount of tension that it can provide is limited to the variation in force provided by the spring.
U.S. Pat. No. 6,953,407 to Mitsubishi Denki Kabushiki Kaisha discloses an automatic belt tension adjuster for setting the tension of a drive belt in a plurality of stages. The tensioner is applied in a vehicle that utilizes a belt alternator starter. The tension adjuster adjusts the tension of the belt such that the belt tension is set to be greater when the engine is started by the alternator than when the alternator is driven to operate after the engine has been started. The tension adjuster includes: a pulley unit around which the belt is wrapped and an automatic belt tensioner for urging the pulley unit to push the belt. The automatic belt tensioner includes a first cylindrical housing with first and second axial end walls; a piston, which is axially movable within the cylindrical housing, the piston having a planar disk portion having an outer diameter equivalent to an inner diameter of the cylindrical housing; a push rod, which is axially movable within the cylindrical housing and extends through one of the axial end walls to contact the pulley unit, the push rod having a planar disk portion with an outer diameter equivalent to an inner diameter of the cylindrical housing; a first elastically deformable spring arranged between the disk portion of the piston and the disk portion of the push rod; and a control unit that axially moves the piston. One of the shortcomings of this device is that the amount of tension that it can provide is limited to the variation in force provided by the spring.
U.S. Pat. No. 7,217,206 to The Gates Corporation discloses an automatic tensioner having a load cell for detecting and controlling a power transmission belt tension. The tensioner includes a lead screw driven by an electric motor for setting a lever arm/pulley position and thereby a belt load. The tensioner also includes a load cell engaged with a tensioner lead screw for detecting a belt load. The tensioner motor is controlled via a PID control loop using a load cell signal. The controller compares a detected belt load from the load cell with a predetermined belt load value to identify a desired belt load and thereby set a tensioner lever arm position corresponding to the desired belt load. One of the shortcomings of this device is that it requires a load cell which adds cost and complexity to the device. The device also does not compliantly tension the belt.